The invention relates to a method and a device for producing hot working gases, in particular for a gas turbine system, with the characteristics of the preamble of claim 1 or, respectively, claim 4.
EP 0 882 486 A1 discloses a method and a device of the above-mentioned type. The known device has a burner that is supplied on an inlet side with fuel and oxygen-containing gas. In the burner, a combustion mixture of the oxygen-containing gas and the fuel is burned, whereby hot combustion waste gas is formed. A waste gas line, through which the hot combustion gas exits the burner and can be used at least in part as hot working gas in a following process, is connected to an outlet side of the burner. The known device also has an oxygen separation device that is supplied at a first inlet with combustion waste gas that is branched off from the waste gas line. At a second inlet, this oxygen separation device is supplied with heated, oxygen-containing gas. The oxygen separation device is provided with oxygen separation means that transport oxygen from the heated, oxygen-containing gas to the branched-off combustion waste gas. Oxygen-enriched combustion waste gas that is used as the oxygen-containing gas for supplying the inlet side of the burner then exits at a first outlet of the oxygen separation device. At a second outlet of the oxygen separation device, hot gas with reduced oxygen content and which can be used in a subsequent process as a hot working gas exits. In the known device, the hot gas with reduced oxygen content is used to heat the oxygen-containing gas fed to the oxygen separation device in a heat exchanger.
In principle, it is also possible, however, to use the hot working gases produced in this way, for example, in a gas turbine system for generating electric energy. By using such a device or such a method, it is possible to significantly reduce the noxious emissions during energy generation, especially CO2 emissions, created during the combustion of fossil fuels.
The core idea of this method and these devices is that pure oxygen is used as an oxidant for the combustion, since this significantly simplifies the waste gas after treatment. The reason for this is that a combustion process with molecular oxygen results in a waste gas that essentially consists only of CO2 and H2O. Since oxygen, which is produced in refrigerated plants, is very expensive, new technologies have been developed for producing oxygen. In this context, oxygen separation devices that are provided with a membrane that conducts oxygen ions and electrons, so-called MCM membranes (mixed conducting membranes play an important role. Such an MCM membrane is provided with a retention side, on which the oxygen-containing gas is located, and a pass-through side, on which the gas to be enriched is located. The MCM membrane transports oxygen ions from the retention side to the pass-through side and causes an electron transport from the pass-through side to the retention side. This causes oxygen to be removed from the gas on the retention side and to be fed to the gas on the pass-through side. In order to increase the efficiency of such an MCM membrane, it is advantageous to set a relatively high flow speed on the pass-through side in order to keep the oxygen concentration on the pass-through side as low as possible. It is advantageous for a long useful life of the MCM membrane to perform the following process steps independently from each other in separate units: heating of the oxygen-containing gas, transport of the oxygen from the oxygen-containing gas to the branched-off combustion waste gas, and combustion of the oxygen-enriched combustion waste gas with fuel. The functional separation of these procedures makes it possible to optimize the individual process steps separately, whereby, in particular, the useful life of the MCM membrane can be increased. In other known devices, described below, the previously mentioned processes are able to take place more or less simultaneously in a so-called membrane reactor that essentially corresponds to an oxygen separation device with MCM membrane, but is operated at substantially higher temperatures.
U.S. Pat. No. 5,976,223 discloses a device for producing carbon dioxide and oxygen that works with two oxygen separation devices that each are equipped with an MCM membrane. The first oxygen separation device, which fictions as a membrane reactor, is supplied with oxygen-containing gas that has been compressed and heated on the retention side. On the retention side, a gaseous fuel is supplied that reacts with the supplied oxygen and forms water and carbon dioxide. The oxygen-containing gas with reduced oxygen content is heated by the exothermic reaction that takes place during this process. The oxygen-containing gas heated in this manner is then fed to the second oxygen separation device on its retention side. The desired oxygen then accumulates on the pass-through side of this second oxygen separation device.
WO 98/55394 describes a method in which an oxygen separation device working with an MCM membrane is used as a membrane reactor for producing hot combustion waste gases for a gas turbine system. Ambient air is hereby compressed, heated, and fed to the retention side of the membrane reactor. A mixture of recycled waste gas and fuel is fed to the pass-through side. In the membrane reactor, oxygen is then deleted from the supplied air and is fed into the mixture. The fuel then reacts on the pass-through side with the oxygen on the membrane surface that is coated with an oxidation catalyst. The hot waste gases formed in this manner are then fed into a turbine.
WO 98/55208 discloses another method for producing hot combustion waste gases for operating a turbine, in which compressed fresh air is heated in a first burner and is fed to the retention side of an oxygen separation device working with an MCM membrane. Recycled waste gas is fed together with fuel into a second burner that may be constructed as a catalyzer. The combustion waste gases produced there are then fed to the pass-through side of the oxygen separation device, where they are enriched with oxygen. The oxygen-enriched waste gases are then fed to a third burner and burned there with fuel in order to produce hot combustion waste gases that drive a turbine.
The invention at hand relates to the objective of disclosing an embodiment for a method and a device of the initially mentioned type with improved efficiency.
In accordance with one embodiment of the present invention, the invention uses the general idea of using the oxygen-enriched, hot combustion waste gases after their exit from the oxygen separation device in order to heat the oxygen-containing gases before these are fed to the oxygen separation device. This measure makes it possible to significantly increase the inlet temperature of the oxygen-containing gases without having to introduce energy from the outside to the system. At the same time, the mass flow for the cooled, oxygen-enriched combustion waste gas can be increased. It is also possible to use standard, mechanically operating compressors or pumps for driving the oxygen-enriched combustion waste gases fed to the burner. By using the energy that is present in any case for heating the oxygen-containing gas, the efficiency of the process can be increased yet further.
In another embodiment of the present invention, the invention uses a heat exchanger, through which oxygen-containing gas to be heated and an oxygen-enriched combustion waste gas exiting from the oxygen separation device are flowing in order to heat the oxygen-containing gases. The desired temperature increase of the oxygen-containing gas is then associated with a useful temperature drop of the oxygen-enriched combustion waste gas. As explained above, this makes it possible to improve the efficiency in producing hot working gases.
In an especially advantageous further development, the oxygen separation device may be provided with a first chamber and a second chamber, whereby the oxygen separation means have a membrane that divides the two chambers from each other and transports oxygen from one chamber into the other chamber, whereby the flow in both chambers has the same direction and flows parallel to the membrane. Correspondingly, the branched-off combustion waste gas and the heated, oxygen-containing gas flow through the oxygen separation device according to the co-current principle. With this construction and this type of operation, a flat temperature profile results in the membrane, both in the flow direction and transversely to it. These characteristics reduce the resulting thermal loads in the membrane, increasing its useful life span.
Other important characteristics and advantages of the invention are found in the secondary claims, drawings, and related descriptions of the drawings.